Lung adenocarcinoma presenting with dual paraneoplastic manifestations: dermatomyositis and Lambert–Eaton myasthenic syndrome

Dermatomyositis (DM) is an autoimmune inflammatory myopathy characterised by proximal muscle weakness and distinctive cutaneous findings, frequently linked to malignancy. Lambert-Eaton myasthenic syndrome (LEMS) is a paraneoplastic disorder of neuromuscular transmission, most commonly associated with small-cell lung carcinoma. We describe a woman in her early 70s presenting with progressive proximal muscle weakness, dysphagia and a characteristic rash. Initial evaluation confirmed DM according to the 2017 European League Against Rheumatism/American College of Rheumatology classification criteria, based on clinical, serological and MRI findings. Persistent weakness and atypical facial involvement prompted further evaluation, revealing electrophysiological evidence of presynaptic neuromuscular junction dysfunction and positive P/Q-type voltage-gated calcium channel antibodies, confirming coexisting LEMS. Chest imaging identified a left upper lobe lesion, and positron emission tomography-computed tomography confirmed metastatic lung adenocarcinoma. This case illustrates the diagnostic complexity of overlapping paraneoplastic syndromes.